The present invention relates generally to measurement probes and more particularly to a combination low capacitance probe tip and socket for a measurement probe.
Voltage measurement probes couple an electrical signal from a device under test to a measurement test instrument, such as an oscilloscope and the like. Measurement probes include a probe head, a transmission line, such as a coaxial cable, and a connector housing having a signal connector, such as a BNC, SMA, BMA connector or the like, that connects to a mating signal connector on the measurement instrument. The probe head generally includes a metal tube or housing in which is disposed a substrate. A probing tip or socket is disposed in a holder that is inserted into one end of the probe head. The probe tip or socket extends from the holder and is electrically coupled to the substrate. The substrate has passive or active circuitry formed thereon that provides high impedance to the circuit generating the signal under test. The substrate circuitry is electrically coupled to the transmission line. The other end of the transmission line is electrically coupled to the signal connector.
High frequency measurement probes, such as those for probing signals in the 3 GHz to 4 GHz range, require high input impedance values and low input capacitances. Generally, these types of probes have an active transistor input stage for low circuit loading and have a frequency bandwidth up to 4 GHZ or more. An example of such a probe is the P6217 Active FET Probe, manufactured and sold by Tektronix, Inc., Beaverton, Oreg. and assignee of the instant application.
FIG. 1 is a side-sectional view of the P6217 probe 10 showing the internal configuration of the probe elements. The probe 10 has an electrically conductive circular tubular housing 12 in which is disposed a substrate 14. The substrate is slightly off center in the tubular housing so that the upper surface of the substrate is centered in the housing. The end of the substrate 14 extends slightly outward from the end of the tubular housing and is tapered to provide clearance for a probe tip holder 16. Passive and active electrical components 18, such as bipolar transistors, resistors, capacitors and the like are mounted on the substrate. A gold foil contact 20 extends over the front end of the substrate 14 for providing an electrical connection between the substrate 14 and a probing tip 22. The probe tip holder 16 is made of insulating material, such as plastic or the like, and is secured within the end of the housing 12. The holder 16 has a cavity 24 with a tapered end 26 that receives a portion of the substrate 14 extending out past the end of the housing 12. The tapered end 26 is centrally formed in the holder 16 and has a bore 28 extending therefrom to the front end of the holder 16. The probing tip 22 has a head 30 and a shank 32 that is disposed in the holder 16 with the head 30 positioned in the tapered end 26 and the shank 32 extending through the bore 28 with the tip of the shank 32 exposed at the front end of the holder 16. A resilient elastomeric member 34 is disposed in the tapered end 26 between the probing tip head 30 and the holder to provide a compressive force between the head 30 and the gold foil contact 20 on the end of the substrate 14. An insulating boot 36 is positioned over a substantial portion of the tubular housing 12 to electrically insulate the housing 12 from the user.
Referring to FIG. 2, there is shown a side-section view of another type of probe 40 having active circuitry. In this particular probe design, a socket 42 is provided in the probe tip holder 44. An example of such a probe is the P6245 Active probe, manufactured and sold by Tektronix, Inc., Beaverton, Oreg. The socket type active probe has a frequency bandwidth up to 1.5 GHz. The probe has a rectangular cross-section electrically conductive housing 46. A substrate 48 having active and passive circuitry 49 mounted thereon is mounted on a carrier 50 that is inserted into the housing 46. A gold foil contact 52 extends over the front end of the substrate 48 for providing an electrical connection between the substrate 48 and the socket 42. The probe tip holder 44 has a bore 54 there through in which is positioned the electrically conductive socket 42. An electrically conductive elastomeric material 56 is positioned between the socket 42 and the edge of the substrate 48 to electrically connect the socket to the circuit board and to provide a compressive force. A second socket 58 is positioned and electrically coupled to the housing 46 to provide a ground connection. An insulating boot 60 is positioned over a substantial portion of the housing 46 to electrically insulate the housing from the user. The socket 42 is designed to allow placement on square pin connectors mounted on a circuit board under test. Also, various types of probe tip adapters are provided with the probe for different types of probing operations. For example, a pointed tip may be inserted into the socket for placing the probe on circuit board traces, IC leads and the like.
A drawback to the socket style tip is that the metal socket diameter is much larger than the needle tip, so the socket is closer to the probe""s metal tubular housing, increasing capacitance. The increased capacitance at the probe tip results in lower frequency bandwidth. Adding the various probe tip adapters further increases the tip capacitance. Likewise, adding a socket tip adapter to the probing tip of the higher frequency probe reduces the overall frequency bandwidth. What is needed is a measurement probe that allows both probing using a probing tip and a socket type connection, without the higher capacitance of a socket type connection.
Accordingly, the present invention is to a low capacitance probe tip and socket for a measurement probe having an insulating plug disposed in one end of a probe head. The insulating plug has a bore there through for receiving a low capacitance probe tip and a recess defining a socket. The socket has an aperture formed therein that provides access to the low capacitance probe tip. An electrically conductive contact is disposed in the aperture that extends into the socket and is in electrical contact with the low capacitance probe tip. In one embodiment, the socket is formed parallel to the low capacitance probe tip. The electrically conductive contact is then a spring contact formed on the probe tip and extends through the aperture into the socket. The spring contact is an electrically conductive beryllium-copper wire affixed to the probe tip and has a fold therein that extends through the aperture into the socket. In a further embodiment, the socket is formed at an angle to the low capacitance probe tip. The electrically conductive contact is then an electrically conductive elastomeric material disposed at the distal end of the socket in electrical contact with the low capacitance probe tip.
A measurement probe has a probe head that includes the low capacitance probe tip and socket. The probe head has a substrate disposed therein with probe circuitry formed on the substrate and an insulating plug disposed in one end of the probe head having a bore there through for receiving a low capacitance probe tip that is electrically coupled to the substrate. The insulating plug has a recess defining a socket with the socket having an aperture formed therein to access the low capacitance probe tip. An electrically conductive contact is disposed in the aperture that extends into the socket and is in electrical contact with the low capacitance probe tip. In one embodiment, the socket is formed parallel to the probe tip and in a further embodiment the socket is formed at an angle to the low capacitance probe tip. In the first embodiment, the electrically conductive element is a spring contact formed on the probe tip and extends through the aperture into the socket. The spring contact is an electrically conductive beryllium-copper wire affixed to the probe tip and has a fold therein that extends through the aperture into the socket. In the further embodiment, the electrically conductive contact is an electrically conductive elastomeric material disposed at the distal end of the socket in electrical contact with the low capacitance probe tip.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with the attached drawings.